mt76: unify sta_rate_tbl_update and related helpers

Use common sta_rate_tbl_update on mt76x0 and mt76x2.
mt76x0 do not have support TPC (transmision power control) implmented,
msta->wcid.max_txpwr_adj is only set for mt76x2.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>